Can you measure the distance between the barrel and inside of freefloat tube on both the right and left side, use a set of calipers or something similar, to see if the barrel is centered inside the freefloat tube?

CY6
Greg Sullivan "Sully"
SLR15 Rifles
TheDefensiveEdge.com
(763) 712-0123

The barrel's bore is not mounted correctly to the center line of the upper receiver.

As stated, could be as simple as debris between the upper barrel socket front face and the back mating edge of the barrel extension,
Or, could be that the upper receiver barrel socket and front face is not square to the upper receiver bore line, or the barrel extension on the barrel not square to the barrel bore line instead.

Checking for debris is easy by just pulling the barrel to check; as well as the latter if you have a lathe.

As for truing the upper receiver barrel socket, I have a Madel that I press into the upper receiver that will true the upper barrel socket channel if it was canted from over torque pressure when the barrel nut was installed.  From there, the upper on Madel is chuck up in the lathe and if the front face of the upper socket needs to be trued, then quickly down as well.



Next comes the barrel and chucked up between two live centers lightly, use a indicator on the back bearing edge of the barrel extension to make sure that is running  true  to the barrel bore.  Also allows you to check to make sure that the barrel is not bent as well.
Here if the barrel extension is not running true to the bore line of the barrel, then becomes a little harder cleaning that up, depending on how bad the run out and where.

Read the full topic, hence all the posts in the topic, then reply......

He is running optics, and on the rig, front sight is mounted to the hand rails.

So with optics mounted to the upper receiver rail and almost out of adjustement range on just this upper, barrel bore is not in line to the center line of the upper receiver,

On the iron sights if they were the problem  and the barrel was mounted indexed with the upper receiver center line and did not have the optic problem to begin with, then the float tube with front sight mounted to it, is mounted casted off the upper receiver center line, or front sight not indexed with the upper receiver TDC index instead.

Seth, I thought you built this uppers, and now wondering why you missed the build up on the barrel extension shoulder when you were putting the uppers together?
If you did not assembly the upper, banking that in the future, you will be assembly all your upper yourself instead.

For a decades now, I Just order  kits with all  un-assemble parts, including the B/C's.
Hence I got tired of wasting time by having to pull everything down so I can assembly everything correctly a second time around (read getting an assembled B/C that the gas key is leaking from the top of the carrier just puts me over the edge every time).

Having seen too many barrels that the chamber/bore/muzzle are not concentric to the OD walls of the barrel, so more of a between centers on the chamber and crown to indicate all the surface of the barrel extension instead.